I did this on my 450D the first time I got dust in it. I had no idea that the focus screen was so delicate. Made it a lot worse before I made it better.

I tried to replace the screen myself (eBay) but I think I made matters worse as I had to resort to sending it to a Canon repair centre. They ended up replacing the entire viewfinder and it cost me over £100.

Lesson learnt though. Any dust that can't be blown off and I'm sending it to a professional for cleaning from now on.

Good luck with yours - hope the screen can be cleaned or swapped easily.

I think I got the screen off eBay for around £10. But seriously, I'd take it to a shop/repair centre to do it. If you get more dust past the screen into the viewfinder it's game over and you'll end up with a bill as big as the one I had.

As far as I know, the focus screen us purely for your reference as to where the camera is focusing. When I looked into how it all worked, I seem to remember the focus system, on the 450D at least, is at the bottom of the camera - I think the mirror is semi-reflective and a small mirror behind the large mirror sends light downwards to the metering and focus system. Of course, I have no idea if this is true of all Canon cameras.

If you're still getting focused shots then I'd say it's not having any impact on the workings of the camera. If you can live with the focus screen looking how it does then it's not an urgent thing to replace. I really messed up mine though so had no choice but to do it - every time I took a photo it reminded me of how stupid I'd been!

I think I still can live with the scratches on my focusing screen, as I live within walking distance of a Canon Service Center. But yeah, you got a point. Every time I look through the viewfinder, those blasted scratches are the first thing I see.

canon rumors FORUM

Had a similar situation with my 5d3 and cleaning the focus screen... At least your screen is user replaceable, mine isn't. In over shooting 10 years professionally, never had anything like this happen to me... Dont sweat it, it happens i guess...

DB

Are you sure that they are scratches? Unusual for micro fiber cloth to scratch.

Try this: remove the lens from the body, use a powerful vacuum cleaner (I've been using a Dyson on my 7D for the last year now), then make a ring with your thumb + forefinger and cover the lens mount with your hand, insert nozzle of vacuum into your hand (so contact is with your skin & not camera body). SUCKs every piece of dust out.

They may just be lines (traces) of dust left by the cleaning cloth, as that part of the viewfinder has ridges (not a flat surface), so it is entirely possible that your cleaning effort has left lines of dust, not scratches.

Are you sure that they are scratches? Unusual for micro fiber cloth to scratch.

Try this: remove the lens from the body, use a powerful vacuum cleaner (I've been using a Dyson on my 7D for the last year now), then make a ring with your thumb + forefinger and cover the lens mount with your hand, insert nozzle of vacuum into your hand (so contact is with your skin & not camera body). SUCKs every piece of dust out.

They may just be lines (traces) of dust left by the cleaning cloth, as that part of the viewfinder has ridges (not a flat surface), so it is entirely possible that your cleaning effort has left lines of dust, not scratches.

One person on my thread when I had issue with my 5d3 viewfinder, someone hinted it could have been something as minor as oils from your fingers or oil transferred to a blower which may have accidentally grazed the focus screen... Really doesn't take much to screw it all up...

Jettatore

My viewfinders are also a mess, but I don't notice while I'm looking through them so other than for re-sale value I could care less. It seems like splotchy oil spots, which is probably exactly what it is, but it's difficult to clean and get perfect, I've tried several times and either I can't quite reach the edges but the center gets cleaned, or it looks fine for a bit but then it dries splotchy again.